And we never even got a chance to say goodbye.
The final piece of evidence that led scientists to conclude the spacecraft had crossed the historic threshold arrived in the form of an other-worldly radio transmission recorded on Voyager's vintage eight-track tape recorder this spring.
"It's the first spacecraft to reach interstellar space," said Ed Stone, a Voyager project scientist at Caltech and former chief of the Jet Propulsion Laboratory in La Cañada Flintridge, where the craft was built. "Think of that. It's really something that's mind-boggling."
Proof of the feat was published Thursday in the journal Science, and focused on two very distinct vibrations that were picked up by Voyager's 30-foot whip antennas.
In what scientists described as a double-stroke of good luck, the antennas were able to convert the density of surrounding space plasma into audio signals — along with the help of two immense and well-timed solar flares.
"We literally jumped out of our seats when we saw the data," said lead study author Don Gurnett, a University of Iowa space physicist and Voyager project scientist. "It was clear that we were in the interstellar medium."
Gurnett and his colleagues concluded that Voyager left the heliosphere — the bubble-shaped region of space dominated by the sun's gusting solar winds — on or around Aug. 25, 2012. It is now 11.6 billion miles from Earth.
Some scientists have referred to the edge of the heliosphere as the boundary of the solar system. But technically speaking, NASA scientists said the solar system extends out to the Oort Cloud, a distant spherical shell that is believed to be the birthplace of many comets.
Voyager 1, which launched in 1977, will not reach the inner edge of the Oort Cloud for another 300 years, and it could take as long as 30,000 years to exit it, according to NASA.
Scientists have been vigorously debating Voyager's whereabouts since March, when it became clear the probe was being bombarded by an increasing number of galactic cosmic rays while the number of high-energy particles emanating from the sun had plummeted.
Stone and other NASA scientists, however, said they could not be certain Voyager had entered interstellar space until the magnetic fields surrounding the craft had changed direction.
After waiting for that change for more than a year, Stone conceded this week that Voyager had yet again proved scientists wrong.
"It's a big surprise, and it's another mystery," Stone said. "This is not what our models were telling us."
Confusion over Voyager's whereabouts has a lot to do with the failure of a specific piece of equipment known as the plasma science experiment, or PLS. The device, which was developed at the Massachusetts Institute of Technology, measures the electron density of space plasma — ionized gas that is ejected from the sun, as well as from other stars.
Cool plasma, the product of stars that exploded millions of years ago, populates interstellar space. It has a high density of about 100,000 electrons per cubic meter of space, Gurnett said.
Super-heated plasma, like the solar wind that flows from our sun, fills the heliosphere. It is much less dense, with only about 1,000 electrons per cubic meter.
A functioning PLS would have been able to sense the density change as Voyager exited the heliosphere.
"The instrument failed in 1980, so the spacecraft is sort of instrument-challenged," Gurnett said. "That's one of the major failures we've had. There really aren't that many."
Voyager does have two functional plasma wave antennas that stretch from its base and form a wide "V." The antennas detect the vibration of excited plasma particles and convert that motion into an audible ringing that is stored on the eight-track tape.